The invention relates to a monolithic mirror objective comprising a lens having a primary mirror on the output surface of the lens and a secondary mirror on the input surface of the lens to form an N-shaped radiation path through the mirror objective. In this case all of the N-shaped radiation path is within the lens and possible further lenses. The invention also relates to an optical arrangement comprising two such mirror objectives.
All standard mirror objectives consist of a primary mirror and a secondary mirror, which may be disposed in connection to one or a number of combined lenses. U.S. Pat. No. 2,378,301 shows such a mirror objective in the form of a camera objective. In a mirror objective of the prior art, the primary mirror images the object in its focal plane, the rays are caught by the secondary mirror and the rays are reflected through an aperture in the primary mirror and into the image plane of the camera of the field glass. The optical axis of the mirror objective coincides with the geometrical axis of the objective which passes through the central part of the objective. The aperture in the primary mirror causes a central obscuration on the entrance pupil of the objective (diaphragming in the centre), that is to say, light cones emanating from each object point become hollowed. In order to reach a given desired strength of light, the outer rays of the cone must be given a large angle with respect to the geometrical axis of the mirror objective. Furthermore, if it is desired to adapt the mirror objective to a small magnification, the diameter of the secondary mirror increases and hence the angle of outer rays increases further. When transferring symbols from a material of, for example, paper to a detector, such an objective is unusable because a depth movement of the material which may be very small results in an unacceptable defocus.